1. Field
This invention relates to audiological screening and testing methods. More particularly, it relates to a phase analysis system method and apparatus employing audiological screening via statistical phase analysis of otoacoustic emissions (OAE's), which are signals that are generated by the hair cells of a functioning inner ear in response to acoustic stimuli as a result of the non-linear properties of the cochlear amplifier.
2. State of the Art
Various audiological screening and testing methods are known. For the hearing testing of infants and small children, measurement systems based on the principle of evoked potentials or otoacoustic emissions via signal averaging and individual evaluation by a tester reviewing various wave pattern hearing responses on a display screen are often employed. Because these types of examinations are expensive, and require skilled evaluators, the results often are dependent upon the subjective skills of the evaluator. To overcome these subjective testing limitations, various signal display and evaluation apparatus, such as Kemp, PCT/GB/00030, published Aug. 7, 1981, entitled "Hearing Faculty Testing Apparatus, were developed. These conventional display methods employ methods of frequency and amplitude analysis of otoacoustic emissions (OAE's) signals generated by the inner ear in response to various tones. The frequency and amplitude of the OAE signals are picked up by a microphone and fed into a wave pattern display for measurement and manual or statistical analysis by an operator, see Zoth, DE 4441127A1 published May 23, 1996. These methods and devices may also employ averaging, where a number of signal intervals following the stimulation ("sweeps") are added synchronously with the stimulus, thus improving the signal-to-noise ratio until the emissions are detectable. These other machines thus require an experienced examiner to make a decision based on a number of objectives and subjective criteria such as the frequency spectrum, typical curve morphology and time distributions of the measured signals. The danger in using a normal averaging procedure is that any waveform can be produced by chance. To guard against this, other commercial systems have added additional features such as correlation of two quasi-simultaneous measurements. While this has proven valuable, the amount of correlation depends on the frequency spectrum of noise and signal, which differs from one measurement to another. As a consequence, a given correlation cannot be used exclusively as the criterion for the presence of emissions.
To measure very small signals in general it is necessary to separate the signal from external noise as well as from artifacts caused by the measurement process. A special case is the detection of a time-domain response of a system that does not have a separate input and output. In this case the stimulus signal is part of the recorded response signal. One of these measurement tasks is the measurement of transient evoked otoacoustic emissions (TEOAE). A short signal is applied to the ear canal and a response is recorded by means of a probe microphone. The recorded signal contains the stimulus as well as the desired emission from the inner ear. In general, the TEOAE-signal is recorded in a region of 4 to 20 ms after the stimulus signal. In certain cases the pure acoustic response of the ear canal and the acoustic coupling assembly can have a decay which is that long. One way to separate this artifact from the desired TEOAE-signal is to take advantage of the non-linear character of OAEs. The TEOAE level is not a linear function of the stimulus level but follows a saturation curve. Pure acoustic responses are linear, which means that the response level is a linear function of the stimulus level. Certain stimulus signals can be used to take advantage of the non-linear character of TEOAEs. This paper describes a special class of stimulus signals that improves the detection accuracy of TEOAE measurements.
Commercial available devices for TEOAE-measurements mostly use a non-linear stimulus of a waveform depicted in FIG. 1. The idea is, to present a impulse or click of a certain amplitude once, then present the same signal with a amplitude factor of -1/3 3 times. If the four response signals are added, the stimulus and linear responses compensate and only non-linear responses, such as TEOAE, remain. These summed frames are usually averaged to gain SNR. FIG. 1 shows commonly used stimulus signal for TEOAE-measurements. The 4 parts of the response are summed which compensates the stimulus along with linear responses of the tested system. The sum is calculated according to s=s(0 . . . 1)+s(1 . . . 2)+s(2 . . . 3)+s(3 . . . 4) where the x-axis values are normalized time-values.
The example in FIG. 1 uses sample rectangular impulses. Clicks or other short signal forms can also be used. The polarity switch between the high impulse and the three low impulses can also be omitted, or be done in the receiver part of the measurement system. Essentially, this polarity switch is what the echo-screen does. The common property all those nonlinear stimuli have is that the amplitude of the single impulses is selected in a way to compensate the stimulus and linear responses at the receiver. Varying gaps can be inserted between the stimulus parts to minimize the influence of periodic external noise. Signal generation and recording have only to be synchronous.
These state-of-the-art signals described above are designed to compensate linear stimulus artifacts, which do quite well in most cases. However, there are some disadvantages of those signals:
as the signals depend on saturation effects of the inner ear, the stimulus level that is required is quite high: The lower amplitude clicks must have a level within the saturation region, the higher ones are about 10 dB stronger (factor of 3). PA1 The speakers that are commonly used have magnetic receivers designed for hearing aids. These magnetic receivers have a strong non-linearity on their own with typical total harmonic distortion (THD) values in the 5% region. This means that the stimulus and decay-artifact suppression does in praxis not work very well. This can cause erratic results as the stimulus artifacts are not separated from TEOAEs. PA1 Separation in the time range: PA1 Separation in the frequency range: PA1 "The phases of the vectors of all signal segments are uniformly distributed over the entire angular range between 0 and 360 degrees." PA1 This uniform distribution corresponds with a purely coincidental noise without reference to the phases of the primary signal. See FIG. 1. PA1 when the significance criterion is met, i.e. if the null hypothesis "No" stimulus synchronous signal present" can be refuted with the previously defined significance level or PA1 if the repeatedly recalculated reference phase angle does not converge within specified limits, or PA1 if the significance level for refuting the null hypothesis does not reach a specified value PA1 after analysis of a specified number of signal segments.
The invention described below provides an improved otoacoustic emissions testing and screening apparatus and method based on a phase analysis system of an analogue signal to provide a "pass"/"fail" or "pass"/"refer" type of response, which is more suitable for automated evaluation without the need for highly trained evaluators.